I can't see any reason why Fish (with blue) wouldn't want to run 4 stifles.
Its a GREAT maindeck card because EVERY deck in the format including the random decks will be doing something stifleable in the first 3 turns. As you point out in the opening post.
- Fetches
- Wasteland
- Bazaar
Right there you hit easily the top 90% of most common decks. Some decks like fish run both Fetch and waste! Even outsiders like Oath or Elves can be suseptable to Stifle. Goblins is a deck of Triggered abilites. Also in a deck of mainly 2cc cards, Stifle can push a 2cc through a Chalice at 2. This is especially powerful if you run answers to chalice that cost 2 mana like Qalsali. Lastly, it can answer answers like Engineered Explosives or Sower.
As you point out Stifle is extremely good in the first few turns of the game, which is why you absolutely want 4, to maximize the stifles in your chances of getting one early.
I disagree that Stifle is less useful late game against Tezz. It IS less useful as mana denial if they have alot of mana - however it has different uses. For 1 it can stifle Tezzeret's Search ability, and they still pay the cost. This can be an issue if they tutured up a

, they won't be able to do that again and have Tezz live. Also Many Tezz decks right now also run a Maindeck Tendrils and often don't include many duresses. The stifles can be very good here as well, at least steal a game from time to time. Finally it can get you a timewalk against and impending TV-Key combo. Which makes it better than many things.
I think Spell Peirce is much more situational than stifle. Every deck is going to give you something to stifle, where Peirce is only good against decks that have little mana and no relevant creatures. I do like spell peirce as a compliment to stifle, because it allows you to do alot of damage to most lines of play with a simple

open. Also with

open, you can use peirce to force someone into cracking a fetch, and follow up with a stifle, therefor getting a solid 2 for 2.